1. Field of the Invention
This invention relates to a method and apparatus for providing protection to a device, apparatus, or object subject to damage under thermally hazardous conditions. For example, the method and apparatus of this invention can be utilized to protect pressurized gas cylinders which, when exposed to high temperatures require some intervention to prevent damage to or destruction of the cylinder. The method and apparatus of this invention are particularly suitable for utilization in wide area applications, that is applications requiring protection from thermally hazardous conditions over a relatively large area, for example fire or over-heating detection in an engine compartment.
2. Description of Prior Art
Numerous devices exist for the protection of objects subject to damage under thermally hazardous conditions. In many cases, these devices act to trigger a safety appliance when the presence of a thermally hazardous condition is detected. For example, thermal pressure relief devices, such as those used with compressed natural gas systems employ fusible alloys which melt when exposed to temperatures which exceed a predetermined threshold. As the fusible alloy melts, it triggers a spring-loaded or pressure-loaded valve, allowing pressure within the system to be relieved, thereby avoiding a possible explosion. Other devices include indirect mechanically actuated pressure relief valves and, in a broader sense, sprinkler systems in buildings which are actuated under conditions of excessive heat generated by fire.
One disadvantage of these known devices is the requirement of an occurrence of a localized thermally hazardous condition in order to activate the device and trigger a desired safety response. For example, in the case of conventional pressure relief devices utilizing fusible alloys, the fusible alloy is an element of the safety device itself. Thus, while thermally hazardous conditions proximate the pressure relief device will cause the fusible alloy to melt, thereby triggering the spring-loaded or pressure-loaded valve, thermally hazardous conditions which occur at a distance from the pressure relief device may not trigger the safety mechanism while still creating a condition which is hazardous to the object being protected by the pressure relief device. For example, conventional pressure relief devices for pressurized gas cylinders are often located at one end of the pressurized cylinder and, thus, heat generated by a fire, for example near a middle portion or at an opposite end of the cylinder, must be conducted through the cylinder and to the pressure relief device before the trigger mechanism functions. Many pressurized gas cylinders are constructed of composite materials which have a relatively low thermal conductivity and, thus, can result in relatively long response times for the pressure relief devices to operate. Excessive heat transfer to the pressurized gas cylinder without releasing pressurized fluid within the cylinder can defeat the purpose of a pressure relief device and result in an explosion.
Remotely operated pressure relief valves are taught, for example, by U.S. Pat. No. 3,974,844. There, a valve device is remotely operated by transmitting electrical power through a wire that deforms from a greater length to a shorter original or memory length when a selected current is applied to the wire heating the wire to a temperature above a transition temperature. When the wire cools, it returns to the greater natural length. The wire is directly connected to a valve stem that is connected to a valve plug.
A shaped memory alloy material is a material which deforms when the material reaches or exceeds a given threshold temperature and returns to its original shape when the temperature of the material drops below the threshold temperature. U.S. patent application Ser. No. 08/687,743 teaches a thermal triggering mechanism for a pressure relief device which is triggered by deformation of a shaped memory alloy element. The shaped memory alloy element in its natural state is elongated and shortens when heated to a set temperature. The shaped memory alloy element may be mechanically linked to another suitable element, such as a rod or cable, for interaction with the closure element of a pressure relief device. However, if the thermally hazardous condition to the device arises at a distance from the shaped memory alloy element, the device being protected may be subjected to excessive thermal conditions without the shaped memory alloy element being heated resulting in the pressure relief device not being activated.